Device made of single-crystal silicon

ABSTRACT

A device made of single-crystal silicon having a first side, a second side which is situated opposite to the first side, and a third side which extends from the first side to the second side, the first side and the second side each extending in a  100  plane of the single-crystal silicon, the third side extending in a first area in a  111  plane of the single-crystal silicon. The third side extends in a second area in a  110  plane of the single-crystal silicon. Furthermore, a production method for producing a device made of single-crystal silicon is described.

CROSS REFERENCE

This application claims the benefit under 35 U.S.C. § 119 of GermanPatent Application No. 102007031549.1 filed on Jul. 6, 2007, which isexpressly incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention is directed to a device made of single-crystalsilicon having a first side, having a second side which is situatedopposite to the first side, having a third side which extends from thefirst side to the second side, the first side and the second side eachextending in a 100 plane of the single-crystal silicon, and the thirdside extending in a first area in a 111 plane of the single-crystalsilicon.

BACKGROUND INFORMATION

In KOH-etching of 100 silicon material, the crystallographic propertiesof the silicon wafer are exploited to etch precisely rectangularopenings in the wafer surface. Vertical walls do not arise in the depth,but rather inclined flanks having an angle of 54.7°. This property isalso a crystallographic property of the 100 wafer material. The inclinedflanks correspond to the 111 planes and actually have an etching stopcharacter in KOH. The 111 planes have the lowest etching rates insilicon crystal by far and may thus be used to provide well-definedopenings having defined inclined walls.

In KOH etching starting from one side of the silicon wafer, walls havingan angle of 54.7° to the 100 plane always result in the depth. Steeperflanks may thus not be implemented from a crystallographic viewpoint.

However, it is desirable in some micromechanical structures to have thesteepest possible flanks to obtain the greatest possible mechanicalstability, or, for example, to be able to place cap wafers having thelargest possible bond area as close as possible to bond pads.

SUMMARY

The present invention is directed to a device made of single-crystalsilicon having a first side, a second side which is situated opposite tothe first side, a third side which extends from the first side to thesecond side, the first side and the second side each extending in a 100plane of the single-crystal silicon, the third side extending in a firstarea in a 111 plane of the single-crystal silicon. In accordance withthe present invention, the third side extends in a second area in a 110plane of the single-crystal silicon. The third side, thus,advantageously has an area which extends perpendicularly to the firstand second sides. A third side (flank) is advantageously, thus, providedwhich is steeper than a conventional third side, which extends onlyalong the 111 plane.

According to an advantageous embodiment of the device according to thepresent invention, the third side extends in a third area in a 111 planeof the single-crystal silicon, the second area being situated betweenthe first area and the third area.

According to an advantageous embodiment of the device according to thepresent invention, the device has a through opening from the first sideto the second side, and the third side is a side wall of the throughopening. The device may advantageously, thus, have trenches or holeshaving steep side walls (flanks).

It may also be advantageous if the through opening has a first openingwidth on the first side and a second opening width on the second side,the first opening width is greater than the second opening width, andthe second opening width is greater than that predefined by a thicknessof the device between the first side and the second side and the angleof the 111 plane of the single-crystal silicon starting from the firstopening width. The second opening width is advantageously greater thanan opening width which would result on the second side if, starting froma through opening on the first wafer side and in consideration of theangle of the 111 etch stop plane to the 100 plane, a 100 silicon waferhaving defined thickness was etched through using KOH etching.

Furthermore, the present invention relates to a production method forproducing a device made of single-crystal silicon, including thefollowing production steps:

-   -   Supplying a substrate made of single-crystal silicon having a        first side, having a second side, which is situated opposite to        the first side, the first side and the second side each        extending in a 100 plane of the single-crystal silicon, the        substrate having a thickness between the first side and the        second side.    -   Providing a first mask, having a first opening with a first        opening width, on the first side of the substrate.    -   Providing a second mask, having a second opening with a second        opening width, on the second side of the substrate, the first        opening and the second opening being situated opposite to one        another, the first opening width being greater than the second        opening width, and the second opening width being greater than        that predefined by the thickness of the substrate and the angle        of the 111 plane of the single-crystal silicon starting from the        first opening width.    -   KOH etching of the substrate from the first and the second side,        a third side being formed, which extends from the first side to        the second side, the third side extending in a first area in a        111 plane of the single-crystal silicon and in a second area in        a 110 plane of the single-crystal silicon.

Through the 2-mask process according to the present invention having KOHetching on both sides, it is possible to produce a third side (etchingflank) having a greater slope than in the related art. As a result ofthe production method, the etching flank no longer extends only alongthe 111 plane of the silicon substrate, but rather partially also alongthe 110 plane, i.e., perpendicular to the first and second sides, whichextend in the 100 plane. An etching flank having a greater slope thusresults on average.

According to an advantageous embodiment of the production methodaccording to the present invention, a third side is formed in the KOHetching production step which extends in a third area in a 111 plane ofthe single-crystal silicon, the second area being situated between thefirst area and the third area.

According to an advantageous embodiment of the production methodaccording to the present invention, the KOH etching of the substrateoccurs simultaneously from the first and the second sides. According toanother advantageous embodiment of the production method according tothe present invention, the KOH etching of the substrate occurs atdifferent times from the first and the second sides. The flank designmay advantageously be controlled by the selection of the particularetching start time and the particular etching times.

By using the “2-mask technique” in the production of KOH-etched passagesin a 100 wafer, it is, thus, possible to be able to implement steeperflanks in silicon wafers, which have only the special feature of notforming a continuous level face. Instead, they have a stepped facehaving an alternating slope, namely having the angles of the 111 and the110 planes.

Through openings having flanks having an adjustable slope mayadvantageously be provided by the present invention in such a way thatthe slope of the flanks generally corresponds to the profile of a bondwedge for the wire bonding, so that the bond wedge may be received withgood fitting precision in the through opening. The flank designaccording to the present invention ensures that the largest possiblebonding surface may be combined with the greatest possible mechanicalstability of the structure around the through opening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional device made of single-crystal silicon.

FIG. 2 shows a preform of a device according to the present inventionmade of single-crystal silicon.

FIG. 3 shows a first embodiment of a device according to the presentinvention made of single-crystal silicon.

FIG. 4 shows a second embodiment of a device according to the presentinvention made of single-crystal silicon.

FIG. 5 shows a method according to the present invention for producing adevice made of single-crystal silicon.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a conventional device made of single-crystal silicon. Amicromechanical device 10, including a single-crystal silicon substrate1, is shown. Micromechanical device 10 has a first side 20 and adiametrically opposite second side 30, which are situated incrystallographic 100 planes of single-crystal silicon substrate 1.Micromechanical device 10 has a thickness 60 between first side 20 anddiametrically opposite second side 30. A recess is etched inmicromechanical device 10 using KOH from first side 20. The recess maybe, as shown, a pocket hole or a through opening (not shown here) fromfirst side 20 to second side 30. A third side 40 is thus provided, whichextends from first side 20 to second side 30.

During KOH etching of 100 silicon material, the crystallographicproperties of the silicon wafer are exploited to etch preciselyrectangular openings in the wafer surface. During the KOH etching,vertical walls do not form in the depth, but rather inclined flankshaving an angle of 54.7°. This property is also a crystallographicproperty of the 100 wafer material. The inclined flanks correspond tothe 111 planes and have an etch stop character during the etching inKOH. The 111 planes have the lowest etching rates in silicon crystal byfar and may thus be used for well-defined openings having definedinclined walls.

Third side 40 therefore runs along a crystallographic 111 plane ofsingle-crystal silicon substrate 1. This plane has an angle of 54.7° tothe 100 plane. Steeper flanks may thus not be implemented from acrystallographic viewpoint.

FIG. 2 shows a preform of an example device according to the presentinvention made of single-crystal silicon. A third side 40 running at asteeper angle than the 54.7° cited may be achieved, i.e., a steeperflank of the through opening, by a 2-mask KOH etching process accordingto the present invention, in which a 100 wafer is etched through fromboth sides. For this purpose, a first mask 70, having a first openingwith a first opening width 52, is applied to first side 20 of siliconsubstrate 1. A second mask 80, having a second opening with a secondopening width 54, is applied to diametrically opposite second side 30 ofsilicon substrate 1. First and second masks 70, 80 are situateddiametrically opposed and must have a defined overlap in a top view. KOHis then used for etching from both sides. FIG. 2 shows the state ofmicromechanical device 10 briefly after etching through siliconsubstrate 1 (e.g., a wafer). As a result of the KOH etching, a throughopening 50 is provided, which is delimited laterally by third sides 40.Through opening 50 has a first opening width 52 on first side 20 ofmicromechanical device 10 and a second opening width 54 on second side30. Distance “b” identifies the overlap of first and second masks 70 and80 here. Third sides 40 each have a first area 42 starting from firstside 20 which, as is conventional, runs along the 111 plane. Third sides40 also each have a tip in the profile shown which is made up of oneedge along a 100 plane and a third area 46 along a further 111 plane asin first area 42 only having a reversed sign of the angle.

FIG. 3 shows a first embodiment of a device according to the presentinvention made of single-crystal silicon. During the progress of KOHetching over time, an etching front additionally arises, starting fromthe formed silicon tip, and is displaced forward in the 110 directioninto silicon substrate 1. This 110 plane perpendicular to the 100 planeforms a second area 44 and travels ever further into silicon substrate 1with progressing etching time.

FIG. 4 shows a second embodiment of the device according to the presentinvention made of single-crystal silicon. This is achieved by continuedKOH etching. Second area 44, which is situated in the 110 plane, isincident on an edge of second mask 80. Third area 46, which is situatedin the 111 plane described above, is no longer present at this moment.Upon continued KOH etching and thus the further advance of second area44, a third area 48 again forms which runs along the 111 plane at thesame angle as first area 42. Second area 44 in the 110 plane thus formsthe connection of first area 42 (of the first 111 plane), starting fromfirst mask 70, and third area 48 (of the second 111 plane), startingfrom second mask 80.

Height h of second area 44 is a function of mask overlap b. The lateralposition of second area 44 in the etching flank is a function of theetching time. This etching behavior is valid only for a mask overlap ofb<d. The flank width projected on the x axis is referred to as d here,which would result if the wafer was completely etched through only fromthe front side. For values of b>d, the first 111 plane practically actsas an etch stop plane, assuming a correspondingly long KOH etching time.The shape of the through etching would then correspond to that ofsingle-side through etching in the related art, having the known 54.7°etching flanks. In other words, first opening width 52 is greater thansecond opening width 54. However, second opening width 54 is greaterthan that predefined by thickness 60 of substrate 1 and the angle of the111 plane of the single-crystal silicon starting from first openingwidth 52 on first side 20 if this 111 plane was continued up to secondside 30.

In one design of the present invention, the etching flanks have aprofile which is generally complementary to the profile of a bondingtool, a bonding wedge, for wire bonding. The through opening is suitablewith a corresponding design of the etching flanks to receive a bondingwedge which has an angle of approximately 30°, for example, in aprecisely fitted manner. A bond pad may be provided on second side 30 inthe area of second opening width 54, which is connectable to a bondingwire from first side 20 using the bonding wedge (not shown).

FIG. 5 schematically shows an example method according to the presentinvention for producing a device made of single-crystal silicon. Themethod has the following production steps:

(A) Supplying a substrate 1 made of single-crystal silicon—having afirst side 20, having a second side 30, which is situated opposite tofirst side 20, first side 20 and second side 30 each extending in a 100plane of the single-crystal silicon, substrate 1 having a thickness 60between first side 20 and second side 30.(B) Providing a first mask 70, having a first opening with a firstopening width 52, on first side 20 of substrate 1.(C) Providing a second mask 80, having a second opening with a secondopening width 54, on second side 30 of substrate 1, the first openingand the second opening being situated diametrically opposite to oneanother, first opening width 52 being greater than second opening width54, and second opening width 54 being greater than that predefined bythickness 60 of substrate 1 and the angle of the 111 plane of thesingle-crystal silicon starting from first opening width 52.(D) KOH etching of substrate 1 from first and second sides 20, 30, athird side 40 being formed, which extends from first side 20 to secondside 30, third side 40 extending in a first area 42 in a 111 plane ofthe single-crystal silicon and in a second area 44 in a 110 plane of thesingle-crystal silicon.

According to one exemplary embodiment, a third side 40 is formed inproduction step (D), which extends in a third area 46, 48 in a 111 planeof the single-crystal silicon, second area 44 being situated betweenfirst area 42 and third area 46, 48.

By using the “2-mask technique” in the production of KOH-etched passagesin a 100 wafer, it is thus possible to be able to implement steeperflanks in silicon wafers which only have the special feature of notforming a continuously level face.

The method may also be performed in a time-controlled manner, i.e., theetching from first and second sides 20, 30 may be performed at the sametime, or at different, predefinable times.

1. A device made of single-crystal silicon having a first side, a secondside, which is situated opposite to the first side, and a third side,which extends from the first side to the second side, the first side andthe second side each extending in a 100 plane of the single-crystalsilicon, the third side extending, in a first area, in a 111 plane ofthe single-crystal silicon, wherein the third side extends, in a secondarea, in a 110 plane of the single-crystal silicon.
 2. The device madeof single-crystal silicon as recited in claim 1, wherein the third sideextends, in a third area, in a 111 plane of the single-crystal silicon,the second area being situated between the first area and the thirdarea.
 3. The device made of single-crystal silicon as recited in claim 1wherein the device has a through opening from the first side to thesecond side, and the third side is a side wall of the through opening.4. The device made of single-crystal silicon as recited in claim 3,wherein the through opening has a first opening width on the first sideand a second opening width on the second side, the first opening widthis greater than the second opening width, and the second opening widthis greater than that predefined by a thickness of the device between thefirst side and the second side and an angle of the 111 plane of thesingle-crystal silicon starting from the first opening width.
 5. Amethod for producing a device made of single-crystal silicon,comprising: (A) supplying a substrate made of single-crystal siliconhaving a first side, and a second side which is situated opposite to thefirst side, the first side and the second side each extending in a 100plane of the single-crystal silicon, the substrate having a thicknessbetween the first side and the second side; (B) providing a first maskhaving a first opening with a first opening width, on the first side ofthe substrate; (C) providing a second mask, having a second opening witha second opening width, on the second side of the substrate, the firstopening and the second opening being situated diametrically opposite toone another, the first opening width being greater than the secondopening width, and the second opening width being greater thanpredefined by the thickness of the substrate and the angle of the 111plane of the single-crystal silicon starting from the first openingwidth; and, (D) KOH etching the substrate from the first and the secondsides, a third side being formed which extends from the first side tothe second side, the third side extending in a first area in a 111 planeof the single-crystal silicon and in the second area in a 110 plane ofthe single-crystal silicon.
 6. The method as recited in claim 5, whereina third side is formed in step (D), which extends in a third area in a111 plane of the single-crystal silicon, the second area being situatedbetween the first area and the third area.
 7. The method as recited inclaim 5, wherein the KOH etching of the substrate occurs simultaneouslyfrom the first and the second sides.
 8. The method as recited in claim5, wherein the KOH etching of the substrate occurs offset in time fromthe first and the second sides.